The present application claims priority to Japanese Application No. P2000-311501 filed Oct. 12, 2000, which application is incorporated herein by reference to the extent permitted by law.
This invention relates to a light control device wherein incident light is transmitted after control in quantity of the light and its driving method, and also to a pickup device using the light control device.
In general, a polarizer is used in a light control device using a liquid crystal cell. For the liquid crystal cell, there is used, for example, a TN (twisted nematic) liquid crystal cell or a guest-host (GH (guest-host)) liquid crystal cell.
FIGS. 13A and 13B are, respectively, a schematic view showing the working principle of a known light control device. This light control device is constituted mainly of a polarizer 1 and a GH cell 2. The GH cell 2 is sealed between two glass substrates and has working electrodes and liquid crystal alignment films (which are not particularly shown herein and whenever the cell is illustrated hereinafter). The GH cell 2 has liquid crystal molecules 3 and dichromatic dye molecules 4 sealed therein.
The dichromatic dye molecules 4 have anisotropy with respect to the absorption of light and are made, for example, of positive-type (p-type) dye molecules that absorb light along the major axis of the molecules. The liquid crystal molecules 3 have dielectric anisotropy of the positive type (p-type), for example.
FIG. 13A shows a state of the GH cell 2 in case where no voltage is applied thereto (or under conditions of applying no voltage). Incident light 5 is linearly polarized after transmission through the polarizer 1. In FIG. 13A, the direction of polarization and the direction of the major axis of the dichromatic dye molecules 4 are coincident with each other, so that the light is absorbed with the dichromatic dye molecules 4, thereby causing the transmittance of the GH cell 2 to be lowered.
When a voltage is applied to the GH cell 2 as shown in FIG. 13B, the liquid crystal molecules are turned toward a direction of an electric field, under which the direction of the major axis of the dichromatic dye molecules 4 becomes perpendicular to the direction of polarization of the linearly polarized light. Thus, the incident light 5 undergoes little absorption with the GH cell 2 and is transmitted.
In the GH cell 2 shown in FIGS. 13A and 13B, as a working voltage is applied thereto, the average transmittance of visible light (in air and when a light transmittance under conditions where a polarizer is used in addition to the liquid crystal cell is taken at a reference (=100%) herein and whenever it appears hereinafter) increases as is particularly shown in FIG. 14. However, a maximum light transmittance in case where a voltage is increased up to 10V is at approximately 60%, with the light transmittance being varied gently.
It will be noted that where negative type (n-type) dichromatic dye molecules capable of absorbing light along the direction of the minor axis of the molecules are used, the light is not absorbed under conditions of applying no voltage, but is absorbed when a voltage is applied thereto, unlike the case using the positive type dichromatic dye molecules 4.
With the light control device shown in FIGS. 13A and 13B, a ratio between the absorbances under voltage-applying conditions and no voltage-applying conditions, i.e. an optical density ratio, is at about 10. This optical density ratio is as high as about two times that of a light control device constituted of the GH cell alone without use of any polarizer 1.
For the drive of the above-stated light control device, drive pulses are changed in a stepwise manner when the light transmittance is changed. However, there arises the problem that depending on the structure of a liquid crystal cell used and the type of liquid crystal material, a response time in case where a light transmittance is slightly changed for half tone becomes much longer than that for a great stepwise response under transparent conditions (i.e. a maximum light transmittance) to light-intercepting conditions (i.e. a minimum light transmittance) or for a great stepwise response under light-intercepting conditions to transparent conditions.
It is accordingly an object of the invention to provide a light control device wherein where a liquid crystal element for light control is driven to realize a half tone, a change or relaxation in orientation of a liquid crystal is allowed to proceed smoothly, so that a response time of a light transmittance is caused to be shortened, thereby improving the performance, image quality and reliability of the light control device.
The invention contemplates to provide a light control device, which comprises a liquid crystal element, and a pulse control unit wherein when a transmittance of light outputted or transmitted from the liquid crystal element is changed from an actual light transmittance to an intended light transmittance, the pulse control unit is able to insert beforehand a drive pulse for control corresponding to a minimum light transmittance or a maximum light transmittance, at least, prior to insertion of a drive pulse corresponding to the intended light transmittance. The invention also contemplates to provide a method for driving the light control device by use of the drive pulse for control and to provide a pickup device having the light control device arranged in a light path of a pickup system.
According to the invention, the light transmittance of the liquid crystal element is slightly changed from an actual light transmittance to an intended light transmittance in a half tone region, a drive pulse for control corresponding to fully light-intercepting conditions (i.e. a minimum light transmittance) or fully transparent conditions (i.e. a maximum light transmittance) is appropriately inserted beforehand prior to application of a drive pulse corresponding to the intended light transmittance. As a result, the change or relaxation in orientation of the liquid crystal proceeds smoothly, so that a response time before the intended light transmittance is reached can be remarkably shortened over the case where a drive pulse corresponding to the intended light transmittance is merely applied to in a stepwise manner so as to drive the cell.